Embryo transfer is a technique whereby a fertilized egg is removed from a female mammal and introduced into the uterus of a second female, where it implants and develops in the normal way. Embryo transfer has become popular as a means for proliferating desirable genotypes, effecting the genetic improvement of food animals, increasing production of food animals, and treating infertility problems. The ova of a desirable female can be fertilized and harvested at each estrus without damage to the animal. Thus, offspring may be procured without the interruption of the production of such ova that would otherwise result from pregnancy in the donor animal.
It is also possible and especially desirable to stimulate the production of several ova at each estrus, effectively multiplying the reproductive capacity of the animal. The induction of multiple follicular growth with subsequent ovulation is referred to as "superovulation." For example, it is an established commercial practice to stimulate the formation of multiple ovarian follicles in cattle by multiple injections of follicle stimulating hormone. Follicle stimulating hormone is prepared from the pituitary glands of slaughtered animals and is a conventional and commercially available material. It is not necessary to use follicle stimulating hormone prepared from specifically bovine pituitary glands in order to stimulate multiple follicular growth in cows. Instead, hormone from pigs, cattle, horses, and the like can be used interchangeably with practical success. Consequently, in the commercial production of follicle stimulating hormone for use with farm animals, no attempt is made to separate such material by species.
Follicle stimulating hormone is a material prepared with attention to its practical effect rather than its purity of precise content. Thus, it would probably be possible to separate out and define a component of commercial follicle stimulating hormone that in fact is responsible for its biological activity. Herein, "follicle stimulating hormone" shall be taken when appropriate to encompass both the commercial preparation and whatever component thereof may be found to be its biologically active ingredient. References to specific amounts shall be to the commercial preparation.
A typical regimen of follicle stimulating hormone treatment in cattle includes a five-day course of injections of the hormone given intramuscularly twice a day to the donor animal just prior to natural or induced estrus. A typical total dose of the hormone is approximately 50 mg divided into ten doses. The largest dose of follicle stimulating hormone is given early, with daily doses decreasing in amount until the total of 50 mg has been given. Other conventional regimens requiring ten innoculations over a five-day period require as much as 73 mg of hormone. See James F. Evans, "Embryo Transfer in Cattle," Large Animal Supplement, Continuing Education Article #8, Vol. II, No. 6 (June, 1980), publ. by Compendium of Continuing Education, p. 591. Thus, it is common for the induction of superovulation to involve ten innoculations over a period of five days of follicle stimulating hormone alone. As a consequence simply of experiencing the needle so often over so short a period, the cow may become cranky, difficult to work with, and even dangerous. In addition, considerable veterinary or technician time is required. Furthermore, the typical minimum dose of approximately 50 mg of follicle stimulating hormone represents a considerable financial investment, as the hormone is an expensive material, in addition to the cost of the veterinary time needed to administer it in multiple injections.
Various attempts have been made to reduce the numbers of injections of follicle stimulating hormone necessary to produce a desirable amount of superovulation. See, for example, C. R. Looney, et al., "Comparison of Follicle Stimulating Hormone (FSH) in Gelatin and Saline Diluents for Superovulating Donor Cattle, " Theriogenology, Vol. 17, No. 1, (January 1982) p. 97. However, follicle stimulating hormone appears to have a relatively short half life in blood serum, making the repeated injections necessary to maintain desirable serum levels over an extended period. Attempts also have been made to avoid repeated injections by incorporating the hormone in various vehicles adapted to release the hormone more slowly into the bloodstream. However, these attempts have not been very successful, as is reported by David A. Morrow, Current Therapy in Theriogenology: Diagnosis, Treatment and Prevention of Reproductive Diseases in Animals, (W. B. Saunders Company: 1980) p. 75. As a consequence, large amounts of follicle stimulating hormone and the multiple injection technique have remained necessary.
Liposomes have been used for entrapment of various materials, including drugs. See for example, Michael W. Fountain, Craig Dees, and Ronald D. Schultz, "Enhanced Intracellular Killing of Staphylococcus aureus by Canine Monocytes Treated with Liposomes Containing Amicacin, Gentamicin, Kanamycin, and Tobramycin," Current Microbiology, Vol. 6 (1981), pp. 373-376. It has not been known to encapsulate follicle stimulating hormones in liposomes in injectable form for time-delayed release of the hormone in cattle.